Use of acid processed ossein gelatin and chain-extended acid processed ossein gelatin as peptizers in the preparation of photographic elements

ABSTRACT

The invention provides a method of nucleating silver halide particles wherein said nucleation is carried out in the presence of acid processed ossein (APO) gelatin or chain-extended acid processed ossein (CE-APO) gelatin and the composition formed therefrom.

This is a Divisional of application Ser. No. 992,301 filed Dec. 21,1992, U.S. Pat. No. 5,378,598.

FIELD OF THE INVENTION

This invention relates to the use of chain-extended acid-processedossein gelatin in the preparation of photographic silver halideemulsions.

BACKGROUND OF THE INVENTION

R-1. T. H. James, "The Theory of the Photographic Process," 4th edition,Macmillan, New York, N.Y. 1977.

R-2. N. Itoh, J. Soc. Photogr. Sci. Tech., Japan, 52, 329 (1989); Y.Toda, in "Photographic Gelatin," H. Amman-Brass and J. Pouradier Ed.,International Working Group for Photographic Gelatin, Fribourg, 1985.

R-3. P. Bagchi, J. Colloid and Interface Sci., 47, 86 (1974).

R-4. M. D. Sterman and J. L. Bello, "Chain Extended Gelatin," U.S.patent application Ser. No. 612,370 filed Nov. 14, 1990.

R-5. P. Bagchi, ACS Symp. Ser., 9, 145 (1975).

R-6. P. Bagchi, J. T. Beck, and L. A. Crede, "Methods of Formation ofStable Dispersions of Photographic Materials," U.S. Pat. No. 4,990,431(1991).

R-7. J. I. Cohen, W. L. Gardner, and A. H. Herz, Advan. Chem. Ser., 45,198 (1975).

R-8. P. Bagchi and S. M. Birnbaum, J. Colloid and Interface Sci., 45,198 (1975).

R-9. H. A. Hoyen and R. M. Cole, J. Colloid Interface Sci., 41, 93(1972).

R-10. R. R. Irani and C. F. Callis, "Particle Size: Measurement,Interpretation and Application," John Wiley, London, 1963.

R-11. B. Chu, "Laser Light Scattering," Academic Press, New York, 1974.

R-12. Anonymous, "Photographic Silver Halide Emulsions, Preparations,Addenda, Processing and Systems," Research Disclosure, 308, pp. 993-1015(1989).

R-13. S. Nagamoto and K. Hori, "Silver Halide Light-Sensitive Material,"U.S. Pat. No. 4,266,010 (1981).

R-14. S. Nagamoto and K. Hori, "Silver Halide Photographic MaterialsWith Surface Layers Comprising Both Alkali and Acid Processed Gelatin,"U.S. Pat. No. 4,021,244 (1977).

R-15. K. Hori and S. Nagamoto, "Photographic Light Sensitive Material,"U.S. Pat. No. 4,201,586 (1980).

R-16. P. Bagchi and W. L. Gardner, "Use of Gelatin-Grafted andCase-Hardened Gelatin Grafted Polymer Particles For Relief From PressureSensitivity of Photographic Products," U.S. Pat. No. 5,026,632 (1991).

Gelatin has been used as the primary peptizer for the precipitation ofsilver halide grains and also as a coating vehicle in conventionalphotographic recording materials for over 120 years and remains one ofthe most important components of photographic systems (R-1, R-2). Cattle(cow) bones are the principal starting material for photographicgelatin. Sometimes, cattle and pig skins are also used. However, skingelatins usually contain photographically active components and,therefore, their uses in photographic systems are limited (R-2). Themanufacture of gelatin involves several stages. The first step is thedeashing process to reduce the calcium (mainly calcium triphosphate orcalcium apatite and calcium carbonate) content of the bones through asoak for about a week in a mineral acid bath. This decalcified materialis referred to as collagen or "ossein."

Collagen or the ossein is a crosslinked and structured polypeptide(R-1), ##STR1## which is further treated either by lime or by a mineralacid to hydrolyze and denature the tertiary, secondary and partly theprimary structures to produce water-soluble gelatin according to theschematics of FIG. 1. During the formation of gelatin collagen, which iscomposed of crosslinked triple helices of α₁ and α₂ chains (MW=285,000),is first denatured to the randomly coiled γ form, then to a mixture ofthe β₁₁ (composed of two α₁ chains MW=190,000), β₁₂ (composed of one α₁and one α₂ chain, MW=190,000), and to single α₁ and α₂ stands(MW=95,000) and sub-alpha fragments (MW<95,000). The solubilized gelatinfractions are leached and, for many applications, deionized by passagethrough ion exchange beds, chilled, noodled, and then dried for storage.Lime processing to produce gelatin requires between 2 to 3 months oftreatment, whereas acid treatment usually needs several days (R-2).Consequently, for a manufacturing procedure, acid processing isdefinitely less expensive compared to lime processing and thuseconomically attractive. However, since acid hydrolysis occurs morerapidly, it is less controllable, and it leads to gelatins that usuallyhave much lower average molecular weights than those derived from limetreatment. As a result, those gelatins may not provide adequate stericstabilization (R-3, R-5) to the emulsion grains. For this reason, anacid processed ossein (APO) gelatin was intermolecularly crosslinked orchain-extended (CE) (R-4) to produce a gelatin sample with viscosity(hence, effective molecular weight) comparable to standardlime-processed gelatin.

Dispersions of silver halide microcrystals (often referred to asemulsions in the photographic literature) with narrow grain-sizedistribution are usually precipitated by the so-called "double-jet"precipitation technique. Those emulsions usually contain gelatin aspeptizer and steric stabilizer, and they make use of solutions of ˜4 MAgNO₃ and 4 M halide salt solutions. Therefore, under precipitationconditions, all electrical double layer effects on the stability of thesilver halide emulsions are virtually negligible. However, the halideion or the silver ion concentration during the precipitation process hasa profound effect on the morphology of the crystals formed during theprecipitation process. In a double-jet precipitation device (FIG. 2, tobe described later and in R-6), the concentration of silver ion (orhalide ion, the two being related by the solubility product of silverhalide) can be measured by a silver electrode and can be maintained atspecific pAg (=-log₁₀ [Ag⁺ ]) values. The particle nucleation and growthprocess that take place at different silver ion or halide ion excesses,produce microcrystals of different morphology. In the case of AgBr, atpBr values greater than 3 generally cubic crystals are obtained, between2 and 3 usually octahedral crystals are formed, and at pBr values below2 platelets or tabular emulsion crystals are usually generated (R-1).However, from the colloid stability point of view, the particle size andcrystal morphology are very important as they both determine the extentand the functional form of the van der Waals' attraction and steric orelectric double layer repulsion (R-3, R-5).

The stability of a sterically stabilized colloidal system is primarilydetermined by the conformation of the adsorbed macromolecule on theparticle surface and the resulting hydrodynamic thickness of theadsorption layer (R-1, R-8). For amphoteric polyelectrolytes, likegelatin and other proteins that exhibit a pH corresponding to zero netcharge (PZC), the extent of the adsorbed amount (F) is usually highly pHdependent (R-8). That phenomenon has been attributed to the ionizationinduced expansion or contraction of isoelectric proteins. Measurementsof the adsorbed layer thickness (L) by inelastic light scattering allowsthe characterization of the colloidal stability criterion of stericallystabilized dispersed systems.

As indicated above, the physical properties of gelatin, such as PZC,molecular weight and molecular weight distribution depend upon thenature of the processing, such as lime or acid. It has generally beennoted that the PZC of lime processed (ossein) gelatin is around pH of4.9 (R-7) and that of acid processed pigskin gelatin is much higheraround pH of 9.1 (R-7). Using a series of acid processed ossein gelatins(APO) that have been processed in acid over 1,3,4 and 10 days, Toda(R-2) demonstrated that the APO gelatins have a much higher PZC and abroader distribution of PZC compared to lime processed gelatins.However, longer acid treatment times lead not only to sharpening of thePZC distribution, but also to movement of PZC to much lower pH. Asindicated above, longer acid treatment produces gelatins with much lowermolecular weight distributions, and those gelatins are generally notconsidered suitable for adequate peptization of silver halide crystalsused in photography. Higher molecular weight (short acid treatment) acidprocessed gelatins (like pigskin gelatin) have been used infrequently inpreparation of photographic emulsions because of the high PZC (R-7).High PZC Ag-halide peptizers under many precipitation conditions (pH andpAg) can lead to sensitized flocculation rather than peptizationdepending upon the PZC (pAg) of the silver halide salt in question. Afurther disadvantage of high PZC gelatins is that they coacervate whenmixed with regular lime processed ossein gelatin under normal coating pHconditions 3-8. Therefore, the cost advantage of acid processed osseingelatins can be exploited only if acid processed gelatins havesufficiently high molecular weight and reasonable low PZC to avoidadverse colloid chemical interactions. Therefore, there is a need for aninvention that will render relatively inexpensive acid processed osseingelatin usable in photographic systems.

Even though acid processed gelatins have not found extensive use inphotographic systems because of the problems outlined above, limiteddisclosure has been located of its use in overcoats, away from thesilver halide containing sensitized layers and in the interlayersbetween the sensitized layer [Nagamoto et al., U.S. Pat. No. 4,266,010(R-13); 4,021,214 (R-14); and Hozi et al., U.S. Pat. No. 4,201,586(R-15)]. It has been reported in those publications that when acidprocessed gelatins, extended acid processed gelatins or acid processedgelatins in combination with standard lime processed gelatins, areutilized to produce photographic overcoat layers, the photographicproducts have greater abrasion resistance. In those disclosures, theterm "extended gelatin" has been defined as

"Gelatin that has been chemically modified by grafting onto it smallmolecules or other polymers, via the gelatin amine, immine or carboxylgroups, to form either a water soluble or water dispersible polymeric orcolloidal product."

Also such gelatins in the prior disclosures are in general acidprocessed skin gelatins as opposed to acid processed ossein gelatins.

PROBLEM TO BE SOLVED BY THE INVENTION

Therefore, there is a desire to lower costs by utilizing inexpensiveacid processed ossein gelatin in preparation of photographic silverhalide emulsions.

SUMMARY OF THE INVENTION

An object of this invention is to overcome disadvantages of priorphotographic compositions.

A further objective of this invention is to reduce the cost ofphotographic emulsions by utilizing acid processed ossein orchain-extended acid processed ossein gelatins in the preparation ofphotographic emulsions.

Another objective of this invention is to create a high molecular weightacid processed ossein gelatin that provides an adequate gelatin layerthickness in Ag-halide emulsions for steric stabilization andpeptization during nucleation and growth of the emulsions tosufficiently large particle size.

These and other objects of this invention are generally achieved by theuse of acid processed ossein and chain-extended acid processed osseingelatins in the precipitation of silver halide photographic emulsions.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 illustrates the schematics for the preparation of gelatin.

FIG. 2 illustrates the schematic of a double jet precipitation device.

FIGS. 3a, 3b, and 3b illustrate the electrophoretic molecular weightdistributions of various gelatins.

FIG. 4 illustrates transmission electron micrographs of AgBr emulsionsprepared with CE-APO gelatin of Example 3 (Examples 9A, 10A, and 11) andAPO gelatin of Example 2 (Examples 9B and 10B).

FIG. 5 illustrates microelectrophoretic charge profiles of variousgelatins as a function of pH.

FIG. 6 illustrates the pH dependence of the adsorption layer thicknessdue to APO and CE-APO gelatins on AgBr surface at low electrolyte (top)and under swamping electrolyte conditions.

FIG. 7 illustrates the electrolyte concentration dependence of theadsorption layer thickness of APO gelatin of Example 2 on AgBr surfaceat pH=3.0 and pBr=3.0.

FIG. 8 illustrates the pH and electrolyte concentration dependence ofthe conformation of gelatin at the silver halide/water interface.

FIG. 9 illustrates the principles of colloid stability of silver halideemulsion particles under high electrolyte precipitation condition.

ADVANTAGEOUS EFFECT OF THE INVENTION

A major advantage is that acid processed ossein gelatin is much lessexpensive compared to standard lime processed ossein gelatin (APO).Therefore, the ability to fabricate components of photographic productsusing such gelatins vastly reduces the cost of such products. Theadditional advantage of using the preferred chain-extended acidprocessed ossein gelatin (CE-APO) is that such gelatins with increasedmolecular weight provide better peptization and colloid stability oflarger silver halide crystals. The APO gelatins and the CE-APO gelatinsof this invention having IEP values (pH=5.5 to 6.5) closer to those oflime processed ossein gelatins (IEP of about pH 4.9) producephotographic emulsions that are completely compatible with limeprocessed ossein gelatin, which is a great advantage, unlike other acidprocessed skin gelatins such as pigskin gelatin, which has an IEP ofabout pH=9.1.

This invention creates a high molecular weight acid processed osseingelatin for the peptization of silver halide emulsions that haveisoelectric pH close to that of standard lime processed ossein gelatin.Therefore, emulsions precipitated using such gelatins are continuouslycompatible with standard lime processed ossein gelatin overconcentration and pH ranges suitable for use in photographic systems.This is an advantage as the chain-extended acid processed ossein gelatinas at least part of the emulsion is cheaper than presently usedmaterials.

DETAILED DESCRIPTION OF THE INVENTION

For the purpose of this invention, we define "chain-extended gelatins"as follows:

"Chain-extended gelatins are gelatin samples that are produced by thechemical linking of chemically similar (but could be different in chainlengths) gelatin molecules to create a higher molecular weight gelatinsample, which are still water soluble as opposed to a "hardened"product."

Again, for the purpose of this invention, we define "cross-linked" asfollows:

"Chemical bonding between two (polymer) molecules, whereas cross-linkingmay be achieved between two dissimilar polymer molecules, for thepurpose of our invention we use this term for cross-linking between twochemically similar gelatin molecules, that may be different in molecularweight. High degree of cross-linking usually renders gelatin insolublein water."

With reference to photographic prior art, the term "hardening" impliesthe following:

"Cross-linking to create close to infinite molecular weight gelatinmatrix in coatings. The product is insoluble in hot, even caustic water.This state is necessary to prevent dissolution of photographic gelatincoatings during processing."

The term "ossein gelatin" used in this invention is restricted to thefollowing definition:

"Gelatin formed from cattle bones."

The gelatin useful for this invention is formed by intermolecularchain-extension of acid processed ossein gelatin. The parentacid-processed ossein gelatins useful for this invention are those thathave isoelectric pH (IEP) values between 5.5 and 6.5. Such APO gelatinsof this invention are characterized by having a viscosity between 4 and6 cP (m P sec) at low shear rates (i.e., below 100 1/sec) at 40° C. of a6.16% solution in distilled water.

The chain-extended APO gelatins suitable for the invention are thosewith IEP value between 5.5 and 6.5. The CE-APO gelatins of thisinvention are characterized by having a viscosity between 7 and 11 cP (mP sec) at low shear rates (i.e., below 100 1/sec) at 40° C. of a 6.16%solution in distilled water. The CE-APO gelatins of this invention areprepared in a controlled manner, such that the final CE-APO product iscompletely water soluble and does not contain any insoluble fraction.

Although the chain-extended acid processed gelatin compositions of theinvention can be prepared from a gelatin solution containing from about6% and about 18% (dry weight) gelatin, it is preferred that the gelatinconcentration range from about 10% to about 15% by weight. Further,while the concentration of the chain-extending agent ranges from about0.25 to about 5 millimoles per 100 grams (dry weight) of gelatin,preferred amounts range from about 1 to about 3 millimoles per 100 gramsof gelatin.

In the process of the invention, the gelatin solution containing thechain-extending agent of the invention is heated at a temperatureranging from about 40° to about 60° C., preferably 40° to about 50° C.,at a pH ranging from 4.5 and 7, preferably 5.4 to 6, for from about 1 toabout 8 hours, preferably for about 2 to about 4 hours. The pH ismonitored and adjusted at the beginning and the end of the process.

The "chain-extension agent" of this invention can be any suitablegelatin cross-linking agent utilized at sufficiently low concentrations.Such chain-extension agents are described fully in references R-1 andR-16 and are incorporated herein by reference. The preferredchain-extension agents, however, are bis-(vinyl sulfonyl) compoundsbecause they produce intermolecular bonds that are stable to hydrolysis,

Any suitable bis-(vinyl sulfonyl) compound can be used in the practiceof the invention. Preferred classes of such suitable materials includethose having the formulae ##STR2## in which m is an integer of from 1 to4, Z is a heteroatom such as oxygen, nitrogen, sulfur, and the like, andR is hydrogen or lower molecular weight alkyl, such as methyl, ethyl,isopropyl, butyl, pentyl, and the like, which groups can, in turn, befurther substituted. Most preferred are bis-(vinyl sulfonyl) methane andbis(vinyl sulfonyl methyl) ether because they produce intermolecularbonds that are stable to hydrolysis.

Preferred chain-extended gelatins of the invention can be used invarious kinds of gelatin photographic emulsions includingorthochromatic, panchromatic, and infrared emulsions, as well as inx-ray and other nonoptically sensitized emulsions. They can be added tothe emulsions before or after the addition of any sensitizing dyes, andthey are effective in sulfur and gold sensitized silver halideemulsions.

The chain-extended gelatins of the invention can be used in bead coatingand curtain coating operations or can otherwise be coated onto a widevariety of supports. Typical supports include those generally employedfor photographic elements including cellulose nitrate film, celluloseacetate film, polyvinyl acetal film, polystyrene film, polyethyleneterephthalate film, and related films or resinous materials as well asglass, paper, metal, wood, and the like. Supports such as paper that arecoated with α-olefin polymers, particularly polymers of α-olefinscontaining 2 to 10 carbon atoms such as, for example, polyethylene,polypropylene, ethylene butene copolymers, and the like can be employed.

The gelatin compositions of the invention can also contain additionaladditives, particularly those known to be beneficial in photographicemulsions such as optical sensitizers, speed increasing materials,plasticizers, and the like, including those disclosed in U.S. Pat. No.3,128,180 which is hereby incorporated herein by reference. Thus, thechain-extended gelatin compositions of the invention can be used inphotographic elements intended for color photography and can containcolor-forming couplers or be used as emulsions to be developed bysolutions containing couplers or other color generating materials oremulsions of the mixed packet type.

Silver halides employed in photographic emulsions of the inventioninclude any of the photographic silver halides such as silver bromide,silver iodide, silver chloride, silver chloroiodide, silver bromoiodide,silver chlorobromide, and the like. The silver halide crystals may becubic, octahedral, or tabular grain. The silver halides used can bethose which form latent images predominantly on the surface of thesilver halide grains or those which form latent internal images.Hardened emulsions of the gelatins of the invention can be used in colorpaper, transparency, color negative, diffusion transfer, orblack-and-white systems.

For color photographic systems the layer will contain appropriatedye-forming coupler dispersions.

The silver halide emulsions of this invention, prepared using CE-APOgelatins include any of the photographic silver halides such as silverbromide, silver chloride, silver iodide, silver chlorobromide, silverbromoiodide, silver chlorobromide, and the like. The silver halidecrystals may be cubic, octahedral, tabular grains (thickness 30 nm andlarger) or of the Lippmann type (R-1). The APO or CE-APO gelatin can bepresent in the emulsion preparation during nucleation or growth or atboth stages of the inventive process.

Any suitable apparatus or reactor can be used to carry out the chainextension reaction of the invention, including any suitable stirring andheating means. Any suitable means available to those skilled in the artcan be used to adjust the pH of the gelatin composition in accordancewith the invention.

Photographic elements can be prepared using the chain-extended gelatinsof the invention. Suitable elements include a support such as apolyester or polyolefin film, a layer of the chain-extended gelatin ofthe invention on the support, and any suitable hardener including thosedisclosed in any of the patents incorporated herein by reference.

The invention is further illustrated but is not intended to be limitedby the following examples. All parts and percentages are by weightunless otherwise indicated:

EXAMPLES 1-8 Preparation and Characteristics of Chain-Extended AcidProcessed Ossein Gelatins

A. Differences in Molecular Weight Distributions Between Lime Processedand Acid Processed Ossein Gelatins

A lime processed deionized ossein gelatin called gelatin of Example 1,and an acid processed ossein gelatin called gelatin of Example 2, wereanalyzed for molecular weight distribution by gel electrophoresisdescribed by Itoh (R-2). The gel used was a Pharmacia (4/30)polyacrylamide material. Both parent gelatins of Examples 1 and 2 wereobtained from Eastman Kodak Company. The electrophoretic molecularweight distributions of the two gelatins are shown in FIG. 3. Thecalibration markers in FIG. 3 are those for the α, β, and γ chains. Thelow shear viscosities at 6.16% gelatin concentration and at 40° C. alongwith the area percents of the weight fractions of the two gelatins ofExamples 1 and 2 are also shown in Table 1. The results of FIG. 3 andTable I clearly indicate that the APO gelatin is definitely of muchlower average molecular weight than the LPO gelatin. This is why the APOgelatins need to be chain-extended for adequate gel-strength andpeptizing action comparable to that exhibited by LPO gelatins.

                                      TABLE I                                     __________________________________________________________________________    Molecular Weight Fractions and Viscosity of Lime Processed Ossein (LPO)       Gelatin of Example-1 (A), Those of Acid Processed Ossein (APO) Gelatin        of                                                                            Example-2 (B) and Those of Chain-Extended Acid Processed Ossein (CE-APO)      Gelatin of Example-3 (C)                                                                          LPO Gelatin of                                                                        APO Gelatin of                                                                        CE-APO Gelatin of                                    Molecular Weight                                                                       Example-1 [A]                                                                         Example-2 [B]                                                                         Example-3 [C]                             Region     Range    Area %  Area %  Area %                                    __________________________________________________________________________    High MW    >285K    37.4    18.4    36.9                                      Beta       190K to 285K                                                                           12.4    14.7    17.2                                      Alpha      95K to 190K                                                                            23.2    16.7    11.3                                      Sub Alpha  <95K     27.0    50.3    34.6                                      Low Shear Viscosity                                                           η.sub.6.16%.sup.40° C. →                                               Whole Sample                                                                           8.2     4.8     8.9                                       (cP or mP*s)                                                                  __________________________________________________________________________

B. Preparation of Chain-Extended Acid Processed Ossein Gelatins andTheir Physical Properties

All chain extension reactions were carried out by reacting a gelatinsolution with bis-vinylsulfonyl methane, ##STR3## at appropriateconcentrations at 45° C. and at a pH value between 5.7 to 6.1, usuallyfor periods of about 4 hours. The gelatin concentration most suitablefor such reactions was found to be around 12.5%. Compound (H-1) wasadded to the reaction mixture using about a 2% solution at dry weightlevels based on dry weight of gelatin, as indicated in Table II. Aftercompletion of the chain-extension reaction, the gelatin samples werechill set at 4° C., noodled and converted to dry form forcharacterization and other emulsion studies.

The viscosity of a 6.16% solution of each gelatin sample was measuredusing a Brookfield viscometer at a shear rate of about 100/sec. Theinherent viscosity, (η), of a 0.25% solution containing 0.10N KNO₃ wasmeasured using a Cannon-Fenske capillary viscometer. -These viscositymeasurements were made at 40.0°±0.02° C. Viscosity data, ash content,and pH data for the CE gelatins are summarized in Table II.Polyacrylamide gel-electro-phoretic molecular weight distributions werealso determined to obtain an estimate of the high molecular weightfraction, i.e., the molecular weight range from about 2.85×10⁵ to about4.0×10⁵ in each of these gelatin samples. The results are also shown inTable II. It is observed, as expected, that the high molecular weightfraction values parallel the inherent viscosity values and the quantityof hardener used.

                                      TABLE II                                    __________________________________________________________________________    Preparation and Physical Characteristics of the CE-APO Gelatins                                        {η}.sub.0.25%.sup.40° C.                                           Gelation                                                                             IEP                                           Gelatin     %     η.sub.6.2%.sup.40° C.                                                     (0.1N  or %    Area %                                Identi-                                                                             Parent                                                                              BVSM  Gelatin                                                                              KNO.sub.3)                                                                           PZC                                                                              Ash  High MW                               fication                                                                            Gelatin                                                                             (BOGW).sup.c                                                                        cP or mP*S                                                                           cP or mP*S                                                                           pH Content.sup.a                                                                      Fraction.sup.b                        __________________________________________________________________________    Example-2                                                                           --    --    4.8    0.289  5.7                                                                              0.15 18.4                                  Example-3                                                                           Example-2                                                                           0.60  8.9    0.419  5.8                                                                              0.18 36.9                                  Example-4                                                                           Example-2                                                                           --    5.2    0.334  6.1                                                                              0.03 16.2                                  Example-5                                                                           Example-4                                                                           0.20  5.1    0.338  6.0                                                                              0.03 21.9                                  Example-6                                                                           "     0.40  5.8    0.379  5.9                                                                              0.05 31.7                                  Example-7                                                                           "     0.60  7.1    0.435  5.9                                                                              0.06 39.9                                  Example-8                                                                           "     0.80  9.7    0.540  5.8                                                                              0.08 60.4                                  __________________________________________________________________________     .sup.a If 100% of ash is CaO (main component of ash), then for 0.2% ash,      100 g of gelatin will contain 0.0035 moles of Ca.sup.++.                      .sup.b From gelelectrophoresis data.                                          .sup.c BOGW = Based on Gelatin Weight.                                   

The parent APO gelatin of Example 2, although deionized, contained anash residue of 0.15% based on the dry weight of the gelatin. In order tofurther reduce the ash content, an additional deionization of thegelatin was carried out by eluting a solution of this gelatin through amixed bed ion exchange column consisting of Amberlite resins IR 100 andIRA 900. That procedure further reduced the ash content to 0.03%, andthe resulting, further-deionized parent APO gelatin was designated asgelatin of Example 4. The ash composition of the parent APO gelatin ofExample 2 is shown in Table III. From Table III, by far the largestcontributor to the ash is CaO, about 40% by weight of the total ash. Asindicated in Table II, a 0.2% ash consisting of all CaO contributes only0.035 moles of Ca⁺⁺ per 100 g of gelatin. That amount of Ca⁺⁺ willcontribute insignificantly to the ionic strengths in all the experimentsfor which ionic strengths were adjusted.

                  TABLE III                                                       ______________________________________                                        Ash Composition of the Parent APO Gelatin of Example-2                        Ash Composition (Total Ash = 0.15%)                                           Element       % Ash   mg/100 g of Sample                                      ______________________________________                                        Aluminum      1.5     2.25                                                    Antimony      0.02    0.03                                                    Barium        0.05    0.075                                                   Boron         0.07    0.105                                                   Calcium       40      60                                                      Chromium      0.06    0.09                                                    Cobalt        0.08    0.12                                                    Copper        0.15    0.225                                                   Iron          2       3                                                       Lead          0.0025  0.00375                                                 Magnesium     6       9                                                       Manganese     0.03    0.045                                                   Molybdenum    0.004   0.006                                                   Nickel        0.3     0.45                                                    Palladium     0.001   0.0015                                                  Phosphorus    4       6                                                       Platinum      0.3     0.45                                                    Silicon       1       1.5                                                     Silver        0.008   0.012                                                   Sodium        20      30                                                      Strontium     0.1     0.15                                                    Tin           0.025   0.0375                                                  Titanium      0.08    0.12                                                    Vanadium      0.007   0.0105                                                  Zinc          0.01    0.015                                                   Zirconium     0.002   0.003                                                   ______________________________________                                    

The point of zero change (PZC) and the isoelectric pH (IEP) aresynonymous for amphoteric polyelectrolytes like proteins and gelatins(R-7). Besides microelectrophoresis, one of the methods for thedetermination of the IEP or the PZC of such proteins is to exhaustivelydeionize such gelatins and then measuring the pH of the resultingsolution (R-7). This was done for all the samples indicated in Table II,according to the deionization procedure indicated earlier. It is seenthat the parent APO gelatins along with the various chain-extendedsamples have IEP values between 5.7 and 6.1. It is inferred from thosedata that within experimental variability the chain-extension processusing H-1, did not alter the IEP of the modified gelatin samples, eventhough up to a threefold increase in the high molecular weight fractionwas achieved.

A comparison of the molecular weight distributions of the LPO gelatin ofExample 1 and that of the APO gelatin of Example 2 is given in FIG. 3.Table I and Table II indicate the following general features:

1) Molecular weight distribution of all the gelatin samples are indeedvery broad, between 1K to 400K.

2) APO gelatins show much lower viscosity compared to LPO gelatins.

3) LPO gelatins have a substantial high molecular weight peak with atypical twin (α1 & α2) peak in the alpha region.

4) APO gelatins have only a very limited high molecular weight regionbut a pronounced subalpha region with no peaks in the alpha region as inthe case of LPO gelatins.

5) The chain-extension procedure described in this invention is able tocreate an APO gelatin with a high molecular weight peak (and viscosity)similar to that of regular LPO gelatins, but the CE-APO gelatins do notshow twin peaks in the alpha region as do the LPO gelatins.

The amino acid compositions of parent APO gelatin and its variouschain-extended analogs were determined by standard procedures¹ and arereported in Table IV. These measured numbers are the means of 7determinations along with the corresponding standard deviations toprovide the degree of precision of these measurements. In column 5 arelisted published amino acid residue numbers¹ for standard LPO gelatins.Comparison of the data for LPO and the parent APO gelatin of Example 2show that their compositions are very similar, a result to be expectedbecause of the similarity of the source material. An importantdifference, on closer examination, is the ORN content. ORN is not anatural gelatin peptide. It is formed by base hydrolysis of ARG duringthe liming process. As indicated in the footnote of Table IV, ORNamounts vary between 1.5 to 3.5% based upon lining conditions and time.The most significant conclusion regarding ORN is that all the APOgelatins show negligible amounts of ORN as a result of the absence oflime treatment. Existence of significant amount of ORN, therefore, is anindication of liming treatment.

                                      TABLE IV                                    __________________________________________________________________________    Amino Acid Composition of APO, LPO and CE-APO Gelatins                                                    Mean   Standard   Residue per                                                 Residue per                                                                          Deviation                                                                           Residue                                                                            100 of                                                      1000 of 7                                                                            of 7 Deter-                                                                         per 1000                                                                           Further                                                                             Residue per                                           Measure-                                                                             minations                                                                           of   Deionized                                                                           1000 of                                               ments of                                                                             of APO                                                                              Standard                                                                           APO   CE-APO                                                APO Gelatin                                                                          Gelatin of                                                                          LPO  Gelatin of                                                                          Gelatin of                Amino Acid                                                                             Structure          of Example-2                                                                         Example-2                                                                           Gelatin.sup.a                                                                      Example-4                                                                           Example-8                 __________________________________________________________________________    Glycind  H.sub.2 NCH.sub.2COOH                                                                            330.74 1.75  347  326.61                                                                              329.41                    (GLY)                                                                         Proline (PRO)                                                                           ##STR4##          122.08 0.67  120  125.70                                                                              124.68                    Alanine (ALA)                                                                           ##STR5##          110.57 0.68  113  109.58                                                                              111.29                    Hydrooxyproline (HYP)                                                                   ##STR6##          103.67 0.60  96   107.64                                                                              106.84                    Glutamic Acid (GLU)                                                                     ##STR7##          73.60  0.64  74   71.69 72.42                     Arginine (ARG)                                                                          ##STR8##          48.85  0.18  48   49.78 48.97                     Aspartic Acid (ASP)                                                                     ##STR9##          43.94  0.73  44   43.97 43.65                     Serine (SER)                                                                            ##STR10##         35.42  0.38  31   35.58 34.18                     Ammonia (NH.sub.3)                                                                      ##STR11##         28.12  1.19  --   3.10  3.05                      Lucine (LEU)                                                                            ##STR12##         25.83  0.64  24   25.29 25.68                     Lysine.sup.d (LYS)                                                                      ##STR13##         25.81  0.18  27   26.32 24.91                     Valine (VAL)                                                                            ##STR14##         17.88  0.71  22   17.16 19.34                     Threonine (THR)                                                                         ##STR15##         17.33  0.30  17   17.27 17.11                     Phenylalanine (PHE)                                                                     ##STR16##         13.23  0.17  12   13.34 13.32                     Isoleucine (ILE)                                                                        ##STR17##         9.95   0.22  12   9.69  10.22                     Hydroxylsine.sup.d (HYL)                                                                ##STR18##         6.39   0.10  5    6.54  5.50                      Methionine (MET)                                                                        ##STR19##         6.10   0.52  4    5.69  5.26                      Histidine.sup.d (HIS)                                                                   ##STR20##         4.35   0.16  4    4.39  3.43                      Tyrosine (TYR)                                                                          ##STR21##         4.11   0.35  1    3.70  3.69                      Ornithine.sup.c (ORN)                                                                   ##STR22##         0.13   0.018 1.5-3.5.sup.B                                                                      0.07  0.10                      __________________________________________________________________________     .sup.a Taken from Ref. 1;                                                     .sup.b This work, absolute values depends upon liming conditions and time     .sup.c Liming indicator;                                                      .sup.d Vinylsulfone hardening site.                                      

The preparation of chain-extended gelatins by reaction with vinylsulfonehardeners, involve reactions with free amine groups (other than theα-amino group that is involved in the peptide bond formation). Thevinylsulfone hardening sites are LYS, HYL, and HIS which contain freeamine groups. To produce a chain-extended APO gelatin that is useful inthe photographic systems, it is important that after the chain-extensionprocess, sufficient quantities of amine groups still remain in order toproduce a hardenable gelatin. Table IV also shows the amino acidcomposition of further ion exchanged APO gelatin of Example 4 comparedto the parent, that of Example 2. It is seen in the comparison that theamino acid compositions of these two samples are identical except forthe NH₃ content, which is expectedly reduced by the ion exchangeprocess. The CE-APO gelatin of Example 8 in which the largest amount of[H-1 @0.8% BOGW] was used is also shown in Table IV. The high level of(H-i) is clearly reflected in the amino acid composition of CE-APOgelatin of Example 8. In comparison with those of the parent APO gelatinof Example 4, it is observed that the amino acid compositions of the twogelatin samples are identical within experimental variability except forthe hardening indicators such as LYS, HYL, and HIS. In the CE-APOgelatin (Example 8), it is seen that LYS, HYL, and HIS contents aresignificantly lower providing evidence of chain extension. However, itis evident that only a fraction of the hardening sites were, in realityutilized to produce chain extension, so that the resulting CE-APOgelatin still contained sufficient hardening sites for utility inphotographic systems.

EXAMPLES 9-11 Preparation and Characterization of Model AgBr EmulsionsUsing CE-APO Gelatin of Example 3

A. Preparation of the Emulsions

The preparations of the AgBr emulsion were carried out by the so-called"double jet precipitation technique"¹,7 (see equipment in FIG. 2) usingCE-APO gelatin of Example 3 and APO gelatin of Example 2. The AgNO₃(Eastman Kodak Company, Emulsion Grade) and NaBr (Eastman Kodak Company,Reagent Grade) stock solutions were made up at 3.5M each and at pH=6.0.Ten grams of the precipitation gelatins was premelted at 45° C. in 1000mL of distilled water at pH=6.0. After dissolution and melting, thetemperature was lowered to 35° C. The gelatin solution was placed in thestirred precipitation kettle (104) of FIG. 2. The constant temperaturebath (136) was filled with water (138) and maintained at 35° C. Theprecipitation kettle was fitted with a stirrer (116) and asilver-ion-sensing electrode (122) with an automatic temperaturecompensation probe (140). The electrode system was attached to a pAgcontroller (120) which measured and controlled the pAg and recorded thevalues on a strip chart recorder (130). The silver ion (102) and halideion solutions (98) were added simultaneously to the kettle (104). TheAg⁺ flow rate was preset to different values to obtain AgBr emulsiongrains of different sizes. Ag⁺ was pumped at the preset flow rates usingpump (112) through tube (114) into the reaction kettle. The Br⁻ waspumped automatically into the precipitation kettle through tube (121) bythe proportionally controlled pump (118), which was controlled by thepAg controller. The pAg controller sensed the pAg of the precipitationchamber and moderated the pumping rate of the Br⁻ solution to maintainthe pAg set at the pAg controller unit. In all precipitations pBr wascontrolled at 3.0. The pAg and pBr of the precipitation chamber isrelated to each other by the solubility product, pK_(sp), relationship(R-9).

    pAg+pBr=pK.sub.sp

Table V shows the precipitation conditions of the AgBr emulsionsprepared using parent APO gelatin of Example 2 and CE-APO gelatin ofExample 3.

Electron photomicrographs of the precipitated emulsions of Examples 9-11are shown in FIG. 4. It is well known that crystal formation incondensation precipitation takes place via steps of first nucleation andthen growth of the crystals (R-7). It is seen in Table V and FIG. 4 thatthe faster the Ag⁺ (and Br⁻) flow rates, the smaller are the AgBrcrystal size, as faster precipitation is dominated by nucleation ratherthan growth. Conversely, slower precipitation with longer run timesproduced larger sized particles, as expected. Visual examination of themicrographs of FIG. 4 indicates that the emulsion crystals are roundedoctahedra (almost spherical) with fairly narrow size distribution andalso that the repeat runs with APO or CE-APO gelatins produced crystalswith very similar particle size and uniformly. The particle size andsize distributions of the emulsions were measured by two methods. Thefirst involved image analysis of between 1000 to 2000 particles in theelectron micrographs. All samples showed distributions that followed thelog-normal law (R-10). The geometric mean diameter, D_(g), forlog-normal distributions are shown for all examples in Table V. D_(g) isdefined as follows: ##EQU1##

                                      TABLE V                                     __________________________________________________________________________    Conditions for the Precipitation of AgBr Emulsions using APO                  and CE-APO Gelatins at pBr = 3; pH = 6 and at 35° C.                   and Their Particle Size Characteristics                                                              Particle Diameter in Microns                                                                Photon Correlation                                              Image Analysis of Electron                                                                  Spectroscopy After                       Emulsion Silver Ion                                                                          Duration of                                                                           Micrograph    Proteolytic Enzyme                       Example  Flo Rate                                                                            Ag.sup.+  (and Br.sup.-)                                                              Geometric                                                                           Geometric Std.                                                                        Treatment of Emulsion                    [Gelatin Used]                                                                         (mL/min)                                                                            Flow (min)                                                                            Mean  Dev..sup.a                                                                            (scattering angle 90°)            __________________________________________________________________________    Example-9A                                                                             20.0  2.86    0.033 1.21    0.0350                                   [CE-APO Gelatin                                                               of Example-3]                                                                 Example-9B                                                                             20.0  2.86    0.034 1.22    0.0367                                   [APO Gelatin of                                                               Example-2]                                                                    Example-10A                                                                            2.50  22.9    0.047 1.19    0.0494                                   [CE-APO Gelatin                                                               of Example-3]                                                                 Example-10B                                                                            2.50  22.9    0.051 1.19    0.0546                                   [APO Gelatin of                                                               Example-2]                                                                    Example-11                                                                             0.312 182.4   0.085 1.10    0.0893                                   [CE-APO Gelatin                                                               of Example-3]                                                                 __________________________________________________________________________     .sup.a Geometric standard deivation is a unitless number and equals 1.00      for monodisperse systems.   where D.sub.i is the diameter of the i.sup.th     particle of a total n particles. It is seen from the data of Table V that     the particle sizes of equivalent runs using APO or CE-APO gelatins are     about the same. The width of the distribution is characterized by the     geometric standard deviation, σ.sub.g, defined as     ##EQU2##     where n.sub.i is the number of particles with diameter D.sub.i,     σ.sub.g is a number without a unit and is unity for perfectly     monodisperse systems. It is seen in Table IV that σ.sub.g values of     all the emulsions indicate reasonably narrow size distributions, and that     the identical runs with APO or CE-APO gelatins produced similar widths of     distributions.

Photon correlation spectroscopy (PCS, R-11) after proteolytic enzymetreatment was another procedure utilized to determine the core diametersof the AgBr microcrystals. It is well known (R-7) that proteolyticenzymes can digest gelatin to the individual amino acids. Thus, such atreatment is expected to remove the peptizing gelatin shell from aroundthe emulsion crystals. The enzyme treatment procedure was as follows:Tenth of a mL of the raw emulsion was added to 10 mL of distilled water.Then was added 0.1 mL of a 10% solution of the proteolytic enzymeTakamine. The mixture was stirred at 45° C. for about 30 minutes priorto PCS measurements at 90° scattering angle. The determined corediameter by PCS is also shown in Table V. Those values are consistentlyabout 5 to 7% larger than the electron microscopic image analysisnumbers. The larger size may result from the fact that PCS provides anintensity average number which is weighted by the 6th power of theparticle diameter, a result which would weight the distribution (thoughnarrow) to larger particles. Differences of 5 to 7% in the particlediameters obtained by the two methods are not unusual. Despite thatsmall difference, both measurements indicated the same trends.

It is concluded in this section that under similar precipitationconditions, CE-APO gelatin produces stable silver halide emulsions thatare similar in particle size and size distribution to those of theparent APO gelatin.

B. Microelectrophoretic Characterization of CE-APO Gelatin Coated AgBrCrystals Using Emulsion of

EXAMPLE 10B

The determination of the microelectrophoretic mobility of protein orgelatin coated particles such as AgBr emulsions as a function of pHprovides a measure of the net particle charge on the protein as afunction of pH and also the PZC or the IEP value (R-7, R-8). Theelectrokinetic mobility profile of APO gelatin of Example 2 wasdetermined using emulsion of Example 10B as described earlier (R-8). Toimprove observations of particle migration, the emulsion was ripened atpBr=50° C. for 2 hours at pH of about 6.0. The cross-hatched points weredetermined using the same gelatin absorbed on BaSO₄ (MCB reagent grade).It is observed in FIG. 5 that the IEP (or PZC) of the APO gelatin is6.1, compared to a value of pH of 5.7 as determined earlier by the mixedbed ion exchange process (R-7). This is a small difference and it issuggested that the two very different procedures of measuring its IEPvalue give essentially the same results. Microelectrophoresis provides amore quantitative change profile over a pH range. Also superimposed onFIG. 5 are earlier data (R-7) of the electorphoretic mobilities of astandard LPO gelatin and that of a non-inventive high IEP acid processedpigskin (APS) gelatin. The measurement conditions for FIG. 5 are: 35°C., pBr=3.0 and potassium nitrate concentration of 0.0038M. It is clearfrom FIG. 5 why LPO and APS gelatin are not compatible in normal pHranges of photographic applications. Between pH values of 5.9 and 9.1,those gelatins are oppositely charged and cause coacervation uponmixing. As a result AP gelatins, even though much less expensive tomanufacture, are not often used in photographic systems. The APO gelatinof Example 2 and other APO and CE-APO gelatins (see Table II) have IEPvalues close to pH of 6 with only a low positive charge region betweenpH of 5.9 and 6.1, thereby enabling the mixing of CE-APO gelatins of ourinvention with LPO gelatins without observable coacervation in this pHrange. Since gelatins are extremely polydispersed macromolecules, theyhave IEP ranges rather than single IEP values (R-2). Therefore, theCE-APO gelatin of this invention, with IEP values within one pH unit ofthat results in much superior compatibility with LPO gelatins than thatof the LPO gelatin, observed for high IEP APO gelatins or APS gelatins.This is a prime advantage of the CE-APO gelatin of our invention. TheCE-APO gelatin precipitated emulsion of Example 10A was mixed with theLPO gelatin of Example 1 at equal gelatin concentrations at pH of 5.5 at40° C., and no coacervation incompatibility was observed indicating anadvantage of this invention. It is expected that the prepared emulsionswith proper chemical and spectral sensitization would demonstratesimilar photographic sensitivity as similarly precipitated AgBr emulsionusing conventional LPO gelatins. It is to be noted that the experimentof FIG. 5 was performed at a constant KNO₃ concentration of 3.8×10⁻³ M.The contribution to ionic strength from the ash content of 0.15% CaO ismuch less than this number. Therefore, adjustment of ionic strength to3.8×10⁻³ M of KNO₃ is sufficient to override any contribution from thegelatin ash contents in the physical measurements presented here.

C. Characterization of the Adsorption Layer Thickness Due to APO andCE-APO Gelatins and Stability Consideration of AgBr EmulsionsPrecipitated Using Such Gelatins

The stability of a colloidal dispersion, using a polymeric material suchas gelatin, is essentially determined by the thickness of the adsorbedpolymer layer around the particles (R-3). The adsorbed polymer layeracts as a steric barrier and prevents the core particles from comingclose enough such that van der Waal's attraction could causeflocculation or coagulation. Therefore, characterization of thethickness of the gelatin layer around the emulsion particles isessential to determine the stability of sterically stabilized systems.The adsorption layer thicknesses of gelatin on the prepared emulsionwere determined by PCS measurements of the hydrodynamic diameter of thegelatin-adsorbed AgBr grains and the core diameter (as indicated inTable V) and subtracting the latter from the former value. Theadsorption layer thickness, L, of APO gelatin of Example 2 and CE-APOgelatin of Example 3 were determined using emulsions of Example 10B andExample 10A respectively under the following conditions:

Scattering Angle: 90°

t=35° C.

[AgBr]=0.0018M

[gel]=% 8.0×10⁻⁵ g/mL

pBr=3.0

KNO=3.8×10⁻³ M

Results are shown in FIG. 6 (top). [FIG. 6 shows the pH dependence ofthe adsorption layer thickness due to APO gelatin of Example 2 on AgBrgrains of Example 10B and that of CE-APO gelatin of Example 3 on AgBrgrains of Example 10A, at potassium nitrate concentration of 0.0038M(top) and 0.2M (bottom)]. It is seen that the gelatin layer thickness isminimum close to the IEP. But, the layer thicknesses are considerablylarger below and above the IEP, as a result of charge expansion of theadsorbed layer. The adsorbed layer is positively charged below the IEPand negatively above the IEP. The charge expansion can be swamped withelectrolyte. FIG. 7 shows the dependence of the adsorption layerthickness on electrolyte (KNO₃) concentration under the followingconditions:

Scattering Angle: 90°

t=35° C.

[AgBr]=0.0018M

[gel]=8.0×10⁻⁵ g/mL

pBr=3.0

of APO gelatin of Example 2 using the emulsion of Example 10B. It isseen that the adsorption layer thickness with the addition of KNO₃shrinks and levels off at a KNO₃ concentration of 0.1M. In other words,the charge expansion effect is swamped at a concentration of 10⁻¹ KNO₃.FIG. 6 (bottom) shows the adsorption layer thicknesses of APO gelatinand CE-APO gelatin as a function of pH under swamping electrolyteconcentration (2×10⁻¹ M KNO₃). The bottom curve of FIG. 6 shows thatunder swamping electrolyte concentrations, the gelatin adsorption layerthickness is independent of pH and has about the same value as that ofthe IEP at low electrolyte concentration. Since actual emulsion makingtakes place at high electrolyte concentrations, the pH independentswamping electrolyte adsorption layer thickness is the determiningcriterion for stability. It is seen in FIG. 6 that the CE-APO gelatinhas a larger adsorption layer thickness than the parent APO gelatin, aresult due to the much higher molecular weight of the CE-APO gelatinthan that of the parent APO gelatin (FIG. 3, Table I). The conditionsunder which the high electrolyte measurements of FIG. 6 were performedare as follows:

Scattering Angle: 90°

t=35° C.

[AgBr]=0.0018M

[gel]=8.0×10⁻⁵ g/mL

pBr=3.0

KNO=0.2M

The compression and expansion of the adsorption layer thickness ofgelatin as a function of electrolyte concentration and pH is explainedin FIG. 8.

FIG. 9 illustrates the stability criterion of sterically stabilizedcolloidal systems (R-3). Typically, the destabilizing van der Waal'sattraction causes colloids to flocculate or agglomerate. Stericrepulsion at the distance of minimum surface separation of H_(o) =2Lprovides colloid stability. The steep potential barrier due to polymeradsorption layer at H_(o) =2L creates a potential minimum, ΔG_(w),called the secondary minimum (R-3). If the depth of the secondaryminimum is smaller than the energy available in Brownian motion (1.5 kT,k=Boltzmann constant and T absolute temperature), the particles bounceback after Brownian collision and provide a stable dispersion. SinceΔG_(w) occurs at H_(o) =2L for CE-APO gelatins and L is larger than thatin APO gelatins (FIG. 7), CE-APO gelatins, because of their largermolecular weight, should provide better colloidal stability than APOgelatin.

The invention has been described in detail with particular reference topreferred embodiments thereof, but it will be understood that variationsand modifications can be effected within the spirit and scope of theinvention.

We claim:
 1. A composition comprising silver halide grain, limeprocessed ossein gelatin, and chain-extended acid processed osseingelatin (APO) wherein said chain-extended ossein gelatin has anisoelectric pH between 5.5 and 6.5, a viscosity between 7 and 11 cP (m Psec) at 40° C. and at a concentration of 6.16% in distilled water at ashear rate below 100 1/sec, said chain-extended ossein gelatin has ahigh molecular weight fraction, having a molecular weight of greaterthan 285,000, of about 18 to about 60 percent weight fraction gelatin byarea said chain-extended acid processed ossein gelatin is chain extendedusing a bis-(vinyl sulfonyl) chain-extension agent of the followingstructure; ##STR23## in which m is an integer of from 1 to 4, Z isoxygen, nitrogen, or sulfur, and R is hydrogen or lower alkyl, and saidchain-extension agent is used in the range of from about 0.25 to about 5millimoles per 100 g of gelatin dry weight.
 2. The composition of claim1 wherein said silver halide grains comprise cubic, octahedral ortabular crystal structures.
 3. The composition of claim 1 wherein saidsilver halide grains comprise silver bromoiodide, silver chloride, orsilver bromide.
 4. The composition of claim 3 wherein saidchain-extension agent comprises bis-(vinyl sulfonyl) methane orbis-(vinyl sulfonyl methyl) ether.
 5. The composition of claim 3 whereinthe chain-extension agent is used to prepare the CE-APO gelatin is arange of from about 1 to about 3 millimoles per 100 g of gelatin dryweight.
 6. The composition of claim 1 wherein R is a lower alkylconsisting of methyl, ethyl, isopropyl, butyl, or pentyl.